Developments in the use and versatility of photometric instruments, as well as other optical apparatus such as for transmission of information, have expanded a need for switching in optical transmission. An example is switching for a spectrophotometer.
Switching devices generally contain at least one movable optical element with a position or orientation that is changed to switch from one configuration to another. An important requirement of switching devices used in spectrophotometric applications is insensitivity of the light throughput to small, unavoidable motions or jiggling that result in changes in the relative positions or orientations of optical elements. Any change in throughput while the switch is at one setting will cause an incorrect change in a measured photometric value. Such change in throughput upon switching away from and then back to a given setting will result in a lack of reproducibility in the measured photometric value.
Recently optical fibers have been adapted to carry radiation to and from a light transmitting sample probe remote from the instrument. The probe may be immersed in the liquid as described, for example, in U.S. Pat. No. 5,077,481 (Hoult), to monitor for any ongoing changes in the liquid. It sometimes is desirable to monitor several samples with the same photometric instrument, in which case switching is needed for selecting samples or probes. Several types of optical switches have been known. For example, U.S. Pat. No. 5,056,886 (Hoult) discloses the use of a concave mirror for switching between fibers arranged in a bundle, the mirror being rotated about a rotation axis spaced from the optical axis to effect switching. This switch requires accurate alignment to maintain and reproduce light throughput.
Rotatable periscopes for switching are taught in U.S. Pat. Nos. 4,626,065 (Mori) and 5,005,934 (Curtiss). With periscopes, output radiation generally is continued in the same direction as the input radiation, but laterally offset. In principle, the light throughput of these are relatively insensitive to small changes in the position of the periscope alignment, but have some sensitivity to rotational changes. Also, a central motor for switching requires space that adds optical path length to the system, thereby increasing the sensitivity to rotational changes. U.S. Pat. No. 4,989,932 (Landa et al) discloses a multiplexer comprising a rotatable barrel containing a pair of 45.degree. mirrors to deflect light passage to, and return light from, a selected pair of optical fibers. This system is more sensitive than the periscope to changes in both position and orientation of the moving optical elements.
A Porro reflector is similar to a periscope except that the two component reflectors are oriented to direct the output light back toward the input source. Porro reflectors are particularly sensitive to small changes in rotational position about an axis perpendicular to both the edge of the Porro reflector and the light beams.
Retroreflectors, particularly corner cube reflectors, are used in optics to redirect light back toward the originating direction. For example U.S. Pat. No. 4,569,569 (Stewart) discloses a coupler for an optical fiber system. With a single lens for transmitting light both to and from a retroreflector, light is passed between optical fibers. One embodiment includes a beam splitter whereby the retroreflector may be rotated to select pairs of outgoing fibers for transmitting a split beam (FIGS. 7 and 8 of the patent).